Composites reinforced with carbon fibers have been used in applications of aircraft, space and sports, because of their excellent mechanical properties and lightweight.
As a typical method of producing such a composite, autoclave molding is known. In the method, a prepreg comprising a sheet composed of carbon fiber bundles each of which is formed with continuous carbon fibers arranged in one direction and a matrix resin being impregnated therewith. The prepregs are piled up in a mold and heated and pressurized in an autoclave, to be made into a composite.
The prepreg used as a substrate to be molded into a composite has an advantage in production of composite having high reliability. However, it also has a disadvantage that it is too stiff to be drapable. Furthermore, the production of a composite using the prepreg has such problems as high cost and low productivity.
For molding a composite at high productivity, injection molding or infusion molding is known. The injection or infusion molding includes, for example, resin transfer molding (RTM). In the resin transfer molding (RTM), a substrate composed of carbon fiber bundles not pre-impregnated with a matrix resin (dry carbon fiber bundles) is placed in a complicatedly shaped mold, and a matrix resin liquid (low viscosity) is injected or infused into the mold, for making the carbon fiber bundles impregnated with the matrix resin.
However, though the injection or infusion molding is excellent in composite productivity, the substrate used (for example, a dry woven fabric) has problems in view of handling properties such that texture slippage is liable to occur (un-stability of form), that the substrate is so less stiff as to allow easy bending, and that layers of the substrate do not adhere to each other when laminated (no tacky property). In addition, since the matrix resin must be low in viscosity, the composite has a problem of being low in mechanical properties, compared with the composite formed with a matrix resin having a high viscosity as used in the above-mentioned prepreg. These problems present a problem that the composite obtained cannot sufficiently exhibit the properties peculiar to carbon fibers and does not have the mechanical properties expected from the properties of the carbon fibers used.
To solve this problem, U.S. Pat. No. 5,071,711 A proposes a technique, in which a thermoplastic-like resin is applied to a fabric composed of reinforcing fibers, for improving the handling properties of the dry woven fabric used as a substrate, and stabilizing the form of the preform used for injection or infusion molding.
Furthermore, Journal of Advanced Materials, Volume 32, No. 3, Jul. 2000, P27–34 or Composites Part A, Volume 32, 2001, P721–729 reports that if a woven fabric is coated with a resin obtained by mixing an epoxy resin and elastomer particles or polyamide 6, for injection or infusion molding, the mechanical properties (such as the interlaminar fracture toughness of Mode II) of the obtained CFRP are improved.
However, the proposal cannot improve, or can improve only insufficiently, mechanical properties, though it can improve the handling properties of the substrate. That is, for example, the very high levels of mechanical properties required for primary or principal structure elements of aircraft cannot be achieved even if a woven fabric or the like is merely coated with a resin, and in the case where the carbon fibers used themselves do not have the necessary properties, the composite obtained using them cannot exhibit necessary mechanical properties (especially the compression strength after impact) either.
Moreover, in the injection or infusion molding methods described in the above-mentioned proposals, since a sheet composed of carbon fiber bundles merely arranged in one direction cannot be handled with the fiber orientation kept as it is in a dry state, a bi-directional woven fabric is used.
However, for example, primary or principal structure elements of aircraft require very high mechanical properties, especially the compression strength after impact and the compression strength after hot-wet conditioning. In a bi-directional woven fabric, carbon fiber bundles form a bi-directional weave structure. Therefore, the amount of reinforcing fibers in each direction is substantially one half. Furthermore, since the warp and the weft are almost equal in fineness or titer, large crimps of carbon fiber bundles are formed at the interlacing points of warp and weft. Because of these problems, the mechanical properties of a prepreg composed of carbon fibers arranged in two directions could be only halves of those of a prepreg composed of carbon fibers arranged in one direction.
That is, even though the required properties of the carbon fibers to be used and the form of the fabric composed of the carbon fibers are especially important factors for exhibiting high mechanical properties, the above-mentioned proposals do not disclose any explanation about such factors at all.
The object of the invention is to solve the problems of the prior art. Particularly, the object of the invention is to provide a carbon fiber reinforced substrate having excellent handling properties in such as stiffness, form stability, drapability and tackiness, to provide a preform formed with the substrate and having good matrix resin permeability, and to provide a composite formed with the preform having excellent mechanical properties in such as the compression strength after impact or the compression strength after hot-wet conditioning, and having also good productivity.